The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In positron emission tomography (PET) imaging, a tracer agent is introduced into the patient, and the physical and bio-molecular properties of the agent cause it to concentrate at specific locations in the patient's body. The tracer emits positrons, resulting in an annihilation event occurs when the positron collides with an electron that produces two gamma rays (at 511 keV) traveling at substantially 180° apart.
PET imaging systems use detectors positioned around the patient to detect coincidence pairs of gamma rays. A ring of detectors can be used in order to detect gamma rays coming from each angle. Thus, a PET scanner can be substantially cylindrical to maximize the capture of the isotropic radiation. A PET scanner can be composed of several thousand individual crystals (e.g., Lutetium Orthosilicate (LYSO) or other scintillating crystals) which are arranged in two-dimensional scintillator arrays that are packaged in modules with photodetectors to measure the light pulses from respective scintillation events. For example, the light from respective elements of a scintillator crystal array can be shared among multiple photomultiplier tubes (PMTs), or the light can be detected by silicon photomultipliers (SiPMs) having a one-to-one correspondence with the elements of a scintillator crystal array.
Tomographic reconstruction can be used to reconstruct the spatial distribution of the tracer. To this end, each detected event is characterized for its energy (i.e., amount of light generated), its location, and its timing. When a coincidence is detected based on the arrival times of two gamma rays, a line can be drawn between the two locations at which the two gamma rays are detected, i.e., the line-of-response (LOR), and the location of the positron-electron annihilation is somewhere along this LOR. The timing information can also be used to narrow down where along the LOR the annihilation likely occurred by determining a statistical distribution along the LOR based on the time-of-flight (TOF) information, e.g., a difference between the arrival times of the two gamma rays and the resolution of the arrival times.
The position information depends on being able to determine either the arrival position or the identity of scintillator crystal at which a given gamma ray was detected. When PMTs are used as the photodetectors, a many-to-few relationship can result in the light from an array of many scintillator crystal elements (e.g., a 10-by-10 array having 100 crystal elements) being detected and readout by a few PMTs (e.g., four PMTs arranged in a square). When there is light sharing among the PMTs, Anger logic/arithmetic can be used to approximately determine/estimate an interaction position based on the ratio between the pulse height/energy measured by the respective PMTs. Using a flood-map calibration, the Anger-logic position space can be segmented according to the respective crystal elements, and the segmentation can be stored as a lookup table.
One advantage of the above-discussed many-to-few relationship is that only a few channels are needed to multiplex and readout signals from a large number of scintillator crystals, simplifying the readout electronics.
Silicon photomultipliers (SiPMs) can have a much smaller detection area than PMTs, making possible a one-to-one coupling between the SiPMs and the respective scintillator crystals. Nevertheless, a reduced number of readouts can still be desirable when using SiPMs for PET detection. The apparatus for reducing the number of readouts would advantageously not sacrifice performance in detecting timing and/or energy information. Accordingly, improved approaches to reduce the number of readouts for SiPM-based pixelated gamma ray detectors are desired.